This invention relates generally to medical instruments and, in particular, to generators that provide radio frequency (RF) energy useful in sealing tissue and vessels during electrosurgical and other procedures.
Electrosurgical generators are employed by surgeons to cut and coagulate the tissue of a patient. High frequency electrical power, which may be also referred to as radio frequency (RF) power or energy, is produced by the electrosurgical generator and applied to the tissue by an electrosurgical tool. Both monopolar and bipolar configurations are commonly used during electrosurgical procedures.
Electrosurgical techniques can be used to seal small diameter blood vessels and vascular bundles. Another application of electrosurgical techniques is in tissue welding, wherein two layers of tissue are grasped and clamped together by a suitable electrosurgical tool while the electrosurgical RF energy is applied. The two layers of tissue are then xe2x80x9cweldedxe2x80x9d together.
At this point it is significant to note that the process of coagulating small vessels is fundamentally different than vessel sealing or tissue welding. For the purposes herein the term coagulation can be defined as a process of desiccating tissue wherein the tissue cells are ruptured and dried. Vessel sealing or tissue welding can both be defined as desiccating tissue by the process of liquefying the collagen in the tissue so that it crosslinks and reforms into a fused mass. Thus, the coagulation of small vessels if generally sufficient to close them, however, larger vessels normally need to be sealed to assure permanent closure.
However, and as employed herein, the term xe2x80x9celectrosurgical desiccationxe2x80x9d is intended to encompass any tissue desiccation procedure, including electrosurgical coagulation, desiccation, vessel sealing, and tissue welding.
One of the problems that can arise from electrosurgical desiccation is undesirable tissue damage due to thermal effects, wherein otherwise healthy tissue surrounding the tissue to which the electrosurgical energy is being applied is thermally damaged by an effect known in the art as xe2x80x9cthermal spreadxe2x80x9d. During the occurrence of thermal spread excess heat from the operative site can be directly conducted to the adjacent tissue, and/or the release of steam from the tissue being treated at the operative site can result in damage to the surrounding tissue.
It can be appreciated that it would be desirable to provide an electrosurgical generator that limited the possibility of the occurrence of thermal spread.
Another problem that can arise with conventional electrosurgical techniques is a buildup of eschar on the electrosurgical tool or instrument. Eschar is a deposit that forms on working surface(s) of the tool, and results from tissue that is electrosurgically desiccated and then charred. One result of the buildup of eschar is a reduction in the effectiveness of the surgical tool. The buildup of eschar on the electrosurgical tool can be reduced if less heat is develop ed at the operative site.
It has been well established that a measurement of the electrical impedance of tissue provides an indication of the state of desiccation of the tissue, and this observation has been utilized in some electrosurgical generators to automatically terminate the generation of electrosurgical power based on a measurement of tissue impedance.
At least two techniques for determining an optimal amount of desiccation are known by those skilled in this art. One technique sets a threshold impedance, and terminates electrosurgical power when the measured tissue impedance crosses the threshold. A second technique terminates the generation of electrosurgical power based on dynamic variations in the tissue impedance.
A discussion of the dynamic variations of tissue impedance can be found in a publication entitled xe2x80x9cAutomatically Controlled Bipolar Electrocoagulationxe2x80x9d, Neurosurgical Review, 7:2-3, pp. 187-190, 1984, by Vallfors and Bergdahl. FIG. 2 of this publication depicts the impedance as a function of time during the heating of a tissue, and the authors reported that the impedance value of tissue was observed to be near to a minimum value at the moment of coagulation. Based on this observation, the authors suggest a micro-computer technique for monitoring the minimum impedance and subsequently terminating the output power to avoid charring the tissue.
Another publication by the same authors, xe2x80x9cStudies on Coagulation and the Development of an Automatic Computerized Bipolar Coagulatorxe2x80x9d, Journal of Neurosurgery, 75:1, pp. 148-151, July 1991, discusses the impedance behavior of tissue and its application to electrosurgical vessel sealing, and reports that the impedance has a minimum value at the moment of coagulation.
The following U.S. Patents are also of interest in this area. U.S. Pat. No. 5,540,684, Hassler, Jr. addresses the problem associated with turning off the RF energy output automatically after the tissue impedance has fallen from a predetermined maximum, subsequently risen from a predetermined minimum and then reached a particular threshold. A storage device records maximum and minimum impedance values, and a circuit determines the threshold. U.S. Pat. No. 5,472,443, Cordis et al., discusses a variation of tissue impedance with temperature, wherein the impedance is shown to fall, and then to rise, as the temperature is increased. FIG. 2 of this patent shows a relatively lower temperature Region A where salts contained in body fluids are believed to dissociate, thereby decreasing the electrical impedance. A relatively next higher temperature Region B is where the water in the tissue boils away, causing the impedance to rise. The next relatively higher temperature Region C is where the tissue becomes charred, which results in a slight lowering of the electrical impedance. U.S. Pat. No. 4,191,188, Belt et al., discloses the use of two timers whose duty cycles are simultaneously and proportionately adjusted so that high frequency signal bursts are constantly centered about the peak power point, regardless of duty cycle variations.
Also of interest is U.S. Pat. No. 5,827,271, Buysse et al., xe2x80x9cEnergy Delivery System for Vessel Sealingxe2x80x9d, which employs a surgical tool capable of grasping a tissue and applying an appropriate amount of closure force to the tissue, and for then conducting electrosurgical energy to the tissue concurrently with the application of the closure force. FIG. 2 of this patent, shown herein as FIG. 1 for depicting the prior art, illustrates a set of power curves which represent the electrosurgical power delivered to the tissue as a function of the tissue impedance. At low impedances, the electrosurgical power is increased by rapidly increasing the output current. The increase in electrosurgical power is terminated when a first impedance breakpoint, labeled as 1, is reached (e.g.  less than 20 ohms). Next, the electrosurgical power is held approximately constant until proteins in the vessels and other tissues have melted. The impedance at which this segment ends varies in accordance with the magnitude of the RMS power. For example, where the maximum RMS power is approximately 125 Watts, the segment (B) ends at about 128 ohms. When a lower power is used (e.g., 75 Watts), the segment (C) may end at an impedance value of 256 ohms. Next, the output power is lowered to less than one half the maximum value, and the lower power delivery is terminated when a second impedance breakpoint is reached (2.048xc3x97103 ohms). Alternatives to using the impedance for determining the second breakpoint are the use of I-V phase angle, or the magnitude of the output current.
Based on the foregoing it should be evident that electrosurgery requires the controlled application of RF energy to an operative tissue site. To achieve successful clinical results during surgery, the electrosurgical generator should produce a controlled output RF signal having an amplitude and wave shape that is applied to the tissue within predetermined operating levels. However, problems can arise during electrosurgery when rapid desiccation of tissue occurs resulting in excess RF levels being applied to the tissue. These excess levels produce less than desirable tissue effects, which can increase thermal spread, or can cause tissue charring and may shred and disintegrate tissue. It would be desirable to provide a system with more controlled output to improve vessel sealing and reduce damage to surrounding tissue. The factors that affect vessel sealing include the surgical instrument utilized, as well as the generator for applying RF energy to the instrument jaws. It has been recognized that the gap between the instrument jaws and the pressure of the jaws against the tissue affect tissue sealing because of their impact on current flow. For example, insufficient pressure or an excessive gap will not supply sufficient energy to the tissue and could result in an inadequate seal.
However, it has also been recognized that the application of RF energy also affects the seal. For example, pulsing of RF energy will improve the seal. This is because the tissue loses moisture as it desiccates and by stopping or significantly lowering the output the generator between pulses, this allows some moisture to return to the tissue for the application of next RF pulse. It has also been recognized by the inventors that varying each pulse dependent on certain parameters is also advantageous in providing an improved seal. Thus, it would be advantageous to provide a vessel sealing system which better controls RF energy and which can be varied at the outset of the procedure to accommodate different tissue structures, and which can further be varied during the procedure itself to accommodate changes in the tissue as it desiccates.
An accommodation for overvoltage clamping is also desirable. In this regard, conventional overvoltage techniques use a means of clamping or clipping the excess overvoltage using avalanche devices such as diodes, zener diodes and transorbs so as to limit the operating levels. In these techniques the excess energy, as well as the forward conduction energy, is absorbed by the protection device and inefficiently dissipated in the form of heat. More advanced prior art techniques actively clamp only the excess energy using a predetermined comparator reference value, but still absorb and dissipate the excess energy in the form of heat.
U.S. Pat. No. 5,594,636 discloses a system for AC to AC power conversion using switched commutation. This system addresses overvoltage conditions which occur during switched commutation by incorporating an active output voltage sensing and clamping using an active clamp voltage regulator which energizes to limit the output. The active clamp switches in a resistive load to dissipate the excess energy caused by the overvoltage condition.
Other patents in this area include U.S. Pat. No. 5,500,616, which discloses an overvoltage clamp circuit, and U.S. Pat. No. 5,596,466, which discloses an isolated half-bridge power module. Both of these patents identify output overvoltage limiting for all power devices, and overvoltage limit protection is provided for power devices by using proportionately scaled zeners to monitor and track the output off voltage of each device to prevent power device failure. The zener device is circuit configured such that it provides feedback to the gate of the power device. When zener avalanche occurs the power device partially turns on, absorbing the excess overvoltage energy in conjunction with the connective load.
Reference can also be had to U.S. Pat. No. 4,646,222 for disclosing an inverter incorporating overvoltage clamping. Overvoltage clamping is provided by using diode clamping devices referenced to DC power sources. The DC power sources provide a predetermined reference voltage to clamp the overvoltage condition, absorbing the excess energy through clamp diodes which dissipate the excess voltage in the form of heat.
It would be advantageous as to provide an electrosurgical generator having improved overvoltage limit and transient energy suppression.
The foregoing and other problems are overcome by methods and apparatus in accordance with embodiments disclosed herein.
An electrosurgical generator includes a controlling data processor that executes software algorithms providing a number of new and useful features. These features preferably include the generation of an initial pulse, that is a low power pulse of RF energy that is used to sense at least one electrical characteristic of the tissue prior to starting an electrosurgical desiccation cycle, such as a tissue sealing cycle. The sensed electrical characteristic is then used as an input into the determination of initial sealing parameters, thereby making the sealing procedure adaptive to the characteristics of the tissue to be sealed. Another feature preferably provided measures the time required for the tissue to begin desiccating, preferably by observing an electrical transient at the beginning of an RF energy pulse, to determine and/or modify further seal parameters. Another preferable feature performs a tissue temperature control function by adjusting the duty cycle of the RF energy pulses applied to the tissue, thereby avoiding the problems that can result from excessive tissue heating. A further preferable feature controllably decreases the RF pulse voltage with each pulse of RF energy so that as the tissue desiccates and shrinks (thereby reducing the spacing between the surgical tool electrodes), arcing between the electrodes is avoided, as is the tissue destruction that may result from uncontrolled arcing. Preferably a Seal Intensity operator control is provided that enables the operator to control the sealing of tissue by varying parameters other than simply the RF power.
The system disclosed herein preferably further provides a unique method for overvoltage limiting and transient energy suppression. An electrosurgical system uses dynamic, real-time automatic detuning of the RF energy delivered to the tissue of interest. More specifically, this technique automatically limits excess output RF voltages by dynamically changing the tuning in a resonant source of RF electrosurgical energy, and by altering the shape of the RF source signal used to develop the output RF signal. The inventive technique limits the excess output transient RF energy by a resonant detuning of the generator. This occurs in a manner which does not clip or significantly distort the generated RF output signal used in a clinical environment for electrosurgical applications.
A method for electrosurgically sealing a tissue, in accordance with this disclosure, preferably includes the steps of (A) applying an initial pulse of RF energy to the tissue, the pulse having characteristics selected so as not to appreciably heat the tissue; (B) measuring a value of at least one electrical characteristic of the tissue in response to the applied first pulse; (C) in accordance with the measured at least one electrical characteristic, determining an initial set of pulse parameters for use during a first RF energy pulse that is applied to the tissue; and (D) varying the pulse parameters of subsequent RF energy pulses individually in accordance with at least one characteristic of an electrical transient that occurs at the beginning of each individual subsequent RF energy pulse. The method terminates the generation of subsequent RF energy pulses based upon a reduction in the output voltage or upon a determination that the electrical transient is absent.
The at least one characteristic that controls the variation of the pulse parameters is preferably a width of the electrical transient that occurs at the beginning of each subsequent RF energy pulse. The initial set of pulse parameters include a magnitude of a starting power and a magnitude of a starting voltage, and the pulse parameters that are varied include a pulse duty cycle and a pulse amplitude. Preferably, the subsequent RF energy pulses are each reduced in amplitude by a controlled amount from a previous RF energy pulse, thereby compensating for a decrease in the spacing between the surgical tool electrodes due to desiccation of the tissue between the electrodes.
The step of determining an initial set of pulse parameters preferably includes a step of using the measured value of at least one electrical characteristic of the tissue to readout the initial set of pulse parameters from an entry in a lookup table.
The step of determining an initial set of pulse parameters may also preferably include a step of reading out the initial set of pulse parameters from an entry in one of a plurality of lookup tables, where the lookup table is selected either manually or automatically, based on the electrosurgical instrument or tool that is being used.
The method also preferably includes a step of modifying predetermined ones of the pulse parameters in accordance with a control input from an operator. The predetermined ones of the pulse parameters that are modified include a pulse power, a pulse starting voltage level, a pulse voltage decay scale factor, and a pulse dwell time.
Preferably a circuit is coupled to the output of the electrosurgical generator for protecting the output against an overvoltage condition, and includes a suppressor that detunes a tuned resonant circuit at the output for reducing a magnitude of a voltage appearing at the output. In accordance with this aspect of the disclosure, the circuit has a capacitance network in parallel with an inductance that forms a portion of the output stage of the generator. A voltage actuated switch, such as a transorb, couples an additional capacitance across the network upon an occurrence of an overvoltage condition, thereby detuning the resonant network and reducing the magnitude of the voltage output.